1) Field of the Invention
The present invention relates to a system for improving a spatial effect of stereo sound or encoded sound, and particularly relates to a system for improving a spatial effect of stereo sound or encoded sound, e.g. sound processed by Dolby Prologic, AC3, THX or Digital Surround, which is suitably applied to a three dimensional stereo sound image processing technique. According to the invention, a spatial effect in stereo sound or encoded sound and background sound of music are emphasized when a three dimensional image sound is reproduced from a stereo signal using only two speakers; getting a xe2x80x9clivexe2x80x9d sound effect.
2) Related Art
As related arts for improving the spatial effect of stereo sound, there have been suggested in U.S. Pat. No. 4,748,669 xe2x80x9cStereo Enhancement Systemxe2x80x9d or in U.S. Pat. No. 4,866,774 xe2x80x9cStereo Enhancement and Directivityxe2x80x9d. In these U.S. patents, a technique to improve the directivity and spatial effect of stereo sound is disclosed. According to the technique, stereo signals are processed in such a manner to improve the directivity and spatial effect of sound that a left side channel signal (L) and a right side channel signal (R) are added together or subtracted from each other to obtain L+R and Lxe2x88x92R signals, the frequencies, phases and gains of these L+R and Lxe2x88x92R signals are suitably varied; then these signals are calculated at left and right side matrix steps.
In these prior arts, it is essentially required to have a signal processing means for processing the L+R and the Lxe2x88x92R signals; the Lxe2x88x92R signal for forming a stereo sound image should be subjected to filters, gain controlling circuits and other calculating circuits in order to obtain a sound which has a three dimensional effect. However, according to the prior arts, the amount of the differential component of the left and right channel signals becomes extremely small in an output signal because the system is constructed such that the frequency of the Lxe2x88x92R signal is processed and then the thus processed signal is calculated in the left and right channels, respectively. Further, there is a problem that the signal component is lacking in the middle frequency range, i.e. in a voice frequency range, and in the low frequency range, since most of the component of such processed signal are distributed in the high frequency range.
While, by adding the L+R signal to the final matrix step via a different signal lines, the sound is controlled so as to be reproduced at a central position of the left and right speakers; then a well balanced sound can be obtained. However, in such a signal processing system, since the original stereo signal is processed in various manners to obtain L+R and Lxe2x88x92R signals and these signals are reconstructed at the matrix steps after getting the frequency compensation, a large amount of the original sound signal is lost while the three dimensional sense of sound may be obtained. Particularly, there is not left a stereo effect any more because the stereo signals could not be separated from each other well by adding the L+R signal (monophonic signal), so that the separation degree of the sound coming from the left and right speakers and the articulation of sound deteriorate in comparison to the stereo effect which is obtained using generic stereo type audio equipment.
According to the general characteristic of a circuit to reproduce a three dimensional image sound, when sound signals are subjected to a stereo processing circuit, the signal deteriorates in the voice frequency range. In addition to this, when the original stereo signals are reconstructed, some of the original sound is lost. Therefore, if a consumer hears such a sound for a long time, he or she may often feel uncomfortable. Furthermore, when the original sound is processed in filters or phase shifters, mutual interference or distortions are generated among the signals. The loss of a remarkable amount of original sound, which causes inconvenience or discomfort to listeners of music, particularly classical music, cannot be prevented by the conventional technique.
The present invention has for its purpose to provide a system for improving a spatial effect of stereo sound or encoded sound, by which the loss of the original sound can be restricted to a minimum and the sense of three dimensional sound image in the reproduced sound may be improved. According to the present invention, the background sound, which is inevitably decreased during the first signal mixing step of a sound recording process, is enhanced when the sound is reproduced, so that a xe2x80x9clivexe2x80x9d sense of sound can be obtained. According to the invention, the circuitry consists such that neither the L+R signals nor the Lxe2x88x92R signals are processed in various circuits as is done in the above-mentioned prior arts; the channel signals are processed, but importance is given to each channel signal independently. Therefore, the unbalance of an acoustic field can be restricted to a minimum and the ratio between signal and noise and the total harmonic distortion can be decreased, so that the loss of the original sound signal becomes smaller and the directivity and spatial effect of sound can be improved and the xe2x80x9clivexe2x80x9d sense of sound is increased.
It is almost impossible to reproduce a beautiful sound extending over the whole frequency range, i.e. covering a low frequency range, a medium frequency range and a high frequency range, by using cheap or medium priced audio equipment, because the quality or performance of the amplifiers or speakers cannot help but be limited in such equipment. However, if the system according to the present invention is applied, it is possible to eliminate such a problem caused by the limitation of the quality or the performance of the amplifier or speakers to some degree. According to the system of the present invention, the output signal has a construction such that a gain characteristic is increased in a low frequency range taking an original sound signal as a leading part, the original signal and a differential component between the left and right side channels signals exist with a ratio of fifty/fifty in the middle frequency range, and a gain characteristic is increased in the higher frequency range taking the differential signal component of the left and right channel signals as a leading part, so that a natural and real sound effect can be produced. It should also be noted that it is possible to improve the sound reproducing characteristic of the audio signal even if cheap or middle priced audio equipment is used.
In other words, the system according to the invention, which has a symmetrical circuit construction so as to suitably process stereo signals, has realized a new concept of a xe2x80x9csurroundxe2x80x9d system where the spatial effect of sound is improved using a differential component between the left and right channel sound signals, while keeping the circuit construction simple so that it can be said that the ratio between signal and noise is not deteriorated.
As will be stated below in detail, the basic construction of the system according to the present invention is to comprise a spatial effect enhancing portion where a spatial image of sound is extracted in a frequency selective manner, a frequency band enhancing portion where the original sound is enhanced in low frequency range and in the middle frequency range, and a channel matrix portion for calculating signals in a matrix manner.